The use of liquid carbon dioxide for producing CO.sub.2 particles and subsequently accelerating it to high speeds for cleaning particles from a substrate is taught by Layden in U.S. Pat. No. 4,962,891.
Lloyd, in U.S. Pat. No. 5,018,667 at columns 5 and 7, teaches the use of multiple nozzles and tapered concentric orifices for controlling the flow of the CO.sub.2 and the snow mixture. This reference seeks to disperse the snow rather than to focus it after exiting the exhaust nozzle.
Hayashi, in U.S. Pat. Nos. 4,631,250 and 4,747,421, discloses the use of liquefied nitrogen (N.sub.2) for cooling a jacket-type peripheral wall defining a sealed cavity in which a flow of CO.sub.2 gas is introduced under pressure. The cooling produced by the cooled peripheral walls causes the CO.sub.2 to change into snow within the chamber. Nitrogen gas is introduced into the chamber at high pressure in order to agitate and carry the CO.sub.2 particles from the chamber at high velocity though a jetting nozzle.
My U.S. Pat. No. 5,405,283 teaches the use of relatively low pressure shop air for transporting and agglomerating CO.sub.2 particles through a converging-diverging nozzle and onto a large workpiece to be cleaned. The larger CO.sub.2 snow/flakes, produced by the enhanced mixing action upstream from the throat, are useful for aggressively cleaning contaminants from a sturdy workpiece, but are not well suited for cleaning contaminants from pressure-sensitive surfaces because the mass and momentum of the CO.sub.2 particles will cause pitting or denting in pressure-sensitive surfaces.
My U.S. Pat. No. 5,390,450 discloses a converging-diverging nozzle for directing CO.sub.2 particles onto a workpiece. The diverging portion of the nozzle is operated in the overexpanded mode for reducing the noise produced by the shockwave by retaining it within the nozzle.
In contrast to these prior art teachings, the present invention utilizes inexpensive components and readily available low pressure shop air for accelerating relatively small CO.sub.2 particles that are injected directly adjacent to the throat section of a converging-diverging nozzle. It is therefore an object of the present invention to utilize low pressure air for transporting and directing the small CO.sub.2 particles against pressure-sensitive surfaces to be cleaned.